Differential scanning calorimetry, the temperature dependence of proton and deuteron lineshapes and spin-lattice relaxation times are reported for (CH3)2NH2ClO4 and its selectively deuterated analogue (CH3)2ND2ClO4. Our caloric data revealed the existence of three different phase modifications in dimethylammonium perchlorate, analogous to mono- and trimethylammonium perchlorate. An analysis of the NMR data enabled identification of the molecular motions occurring in the respective phases and to determine their activation parameters. In the low-temperature phase III stable below 309 K there are two inequivalent dimethylammonium cations which manifest themselves by different reorientation frequencies of methyl groups about their threefold symmetry axes C3 and by reorientation of one of the two inequivalent cations about its pseudo threefold symmetry axis C'3 lying along one of the two N[BOND]D bonds. In the low-temperature range of this phase the deuteron quadrupole coupling constant indicates N[BOND]H…O hydrogen bonds between the dimethylammonium and the perchlorate ions. The intermediate phase II has not been reported so far. It is stable in the temperature range 309 - 311 K, but can easily be undercooled below 290 K. In this phase one of the two cations undergoes reorientation about its diad C2 axis, whereas the other still reorients about the C'3 axis. The analysis of the large entropy change of 25.5 J/mol · K associated with the III−II transition indicates that the ClO4 ions in phase II have considerable motional freedom, presumably due to breaking of N [BOND] H… O hydrogen bonds. In phase I the dimethylammonium cations undergo quasiisotropic reorientation about their centers of gravity and translational diffusion between different sites of the tetragonal unit cell.